Crocodiles are apex predators that have thrived for millions of years. Their armored bodies, powerful jaws, and stealthy hunting techniques make them formidable creatures. But how do these semiaquatic reptiles breathe underwater?

If you’re short on time, here’s a quick answer to your question: No, crocodiles do not have gills. They are air-breathing animals that can hold their breath for extended periods while submerged.

In this comprehensive article, we’ll explore the respiratory system of crocodilians in depth. We’ll cover the anatomy behind their ability to stay underwater for so long, compare them to other reptiles, and explain how their lungs and cardiovascular system allow them to be such successful aquatic hunters.

The Amphibious Lifestyle of Crocodilians

Semiaquatic Habitats

Crocodilians lead an amphibious lifestyle, meaning they spend time both on land and in water. They inhabit swamps, marshes, lakes, rivers and other bodies of water that provide access to land for basking and nesting.

These semiaquatic habitats allow them to stay cool in the water, while warming their bodies in the sun when out of water.

Time Spent On Land and In Water

Crocodilians may spend over 90% of their time in water, but also require time on land. Hatchlings and younger crocodilians tend to spend more time in water to avoid predators. Adults will spend hours basking to raise their body temperature before hunting at night.

Basking also aids in digestion and egg incubation. Females leave the water to build nests and lay eggs on land. So while water makes up their primary habitat, land is also essential.

Differences Between Crocodiles, Alligators, Caimans, and Gharials

While they belong to the same order of reptiles, there are distinguishing traits between crocodiles, alligators, caimans and gharials:

  • Crocodiles have narrow, V-shaped snouts, while alligators have wider, U-shaped snouts.
  • Caimans are smaller on average and found in Central and South America, unlike alligators which only live in the US and China.
  • Gharials are a slender-snouted crocodilian from the Indian subcontinent, specialized for fish-eating.
Species Average Size Geographic Location
Crocodiles Largest species, up to 20 feet long Africa, Americas, Asia, Australia
Alligators Up to 15 feet long Southeast US, Eastern China
Caimans Usually less than 10 feet long Central & South America
Gharials Up to 20 feet long Northern India, Nepal, Bangladesh

Crocodilian Respiratory Structures and Mechanisms

Nasal Passages and Choanae

Crocodilians have paired external nares or nostrils located at the tip of their snout which lead back into the nasal cavities. These passages are complex, with convolutions and turbinates that increase the mucosal surface area for heat and moisture exchange when breathing.

The posterior nasal openings are called the choanae, which connect to the mouth and throat.

Larynx and Trachea

Air inhaled through the nostrils passes through the larynx, also called the voice box, located at the base of the throat. Crocodilians lack vocal cords but the larynx contains protective valves. The trachea or windpipe branches to send air to each lung.

In crocodilians, the trachea is flattened dorsoventrally with complete tracheal rings, an adaptation for aquatic life.

Lungs

The crocodilian respiratory system features multichambered lungs located in the thoracic cavity. The lungs are attached to the ribs and sternum. Unidirectional airflow occurs through variously-sized parallel tubes in a network of branching bronchi.

The bronchial tree terminates in small sacs called alveoli where gas exchange occurs across a single-cell blood-gas membrane.

Unidirectional Airflow and Aerobic Capacity

Crocodilians utilize a unidirectional airflow pattern during both inspiration and expiration for efficient gas exchange. However, the cardiovascular and respiratory systems limit their aerobic capacity compared to endothermic mammals and birds.

While crocodilians can sustain moderate activity, their blood cannot transport enough oxygen to support prolonged vigorous exertion.

The Hepatic Piston Pump

An unusual aspect of crocodilian respiration is the role of the hepatic piston pump. The liver is attached to the sternum and as this moves during lung ventilation it “pumps” blood back to the heart, enhancing circulation.

Together the cardiac shunt and hepatic pump give crocodilians cardiovascular advantages for holding their breath and diving underwater.

Adaptations That Allow Extended Breath-Holding

Valvular Nostrils and Palatal Flap

Crocodilians have special adaptations that allow them to hold their breath underwater for extended periods while hunting. One of these is valvular nostrils that can close tightly to prevent water from entering.

Located at the very tip of the snout, the nostrils snap shut when the crocodilian dives below the surface. This keeps water out and prevents precious air from escaping.

Crocodilians also have a stiff tissue flap called the palatal flap attached to the roof of their mouth. When closed, this flap seals off the throat from the mouth, adding an extra layer of protection to prevent water from getting into the trachea while submerged.

Slow Metabolism

Another key adaptation is an extremely slow metabolic rate, which means crocodilians use oxygen very efficiently. Their bodies can get by on a fraction of the oxygen mammals like humans need. This allows oxygen stores to last longer on a single breath, permitting stays of 20 minutes or more underwater with just one gulp of air.

Scientists have found the metabolic rates of crocodilians like alligators and crocodiles to be the lowest recorded for any animal. For context, studies show an American alligator’s metabolism is around 10 times slower than a mammal of comparable size.

Bradycardia

Crocodilians can also slow their heart rate tremendously while diving. This condition is called bradycardia. As the crocodilian descends below the water, its heart shifts into a lower gear, beating much more slowly to conserve the circulatory system’s oxygen stores.

Scientists have recorded heart rates as low as 3 beats per minute in diving crocodilians.

In combination with other adaptations like sealed nostrils and extreme energy efficiency, this dive response allows some crocodilian species to stay submerged for hours, waiting patiently to ambush prey.

When they finally surface, it only takes a few gulps of air to replenish their oxygen supply and prepare for another extended underwater stint.

Comparison to Other Reptiles and Amphibians

Lizards and Snakes

Unlike crocodiles, most lizards and snakes do not have a diaphragm or a four-chambered heart. Instead, they rely on more primitive respiration systems. Contractions of muscles near the lungs allow lizards and snakes to breathe.

The heart has three chambers and two aortas that transport oxygen-poor and oxygen-rich blood, limiting efficient circulation. So while crocodiles evolved a respiration system similar to mammals and birds, other reptiles lag behind.

Turtles and Tortoises

Turtles, terrapins, and tortoises have lungs, but lack both a diaphragm and the four-chambered heart of crocodilians. Their unique shell also impacts respiration. To breathe, turtles must extend their heads and legs from the shell which requires considerable energy expenditure.

Sea turtles have adaptations for deep diving including vasoconstriction to ration oxygen use, but all species are aerobic respiration dependent. Of reptiles, only crocodiles rival the efficient respiration of warm-blooded animals.

Frogs and Salamanders

As amphibians, frogs and salamanders have less advanced respiration systems than reptiles. They lack lungs entirely in their larval stages, instead using gills to extract oxygen from water. After metamorphosis, two lungs form but remain simple sacs without complex chambers or a diaphragm.

While frogs and salamanders breathe air as adults, they are not as efficient gas exchangers compared to reptiles. Unlike crocodiles’ advanced cardio-pulmonary design, amphibians demonstrate only modest evolutionary progress in respiration.

Crocodilians Other Reptiles Amphibians
Diaphragm Yes No No
Heart Chambers 4 3 3
Gas Exchange Efficiency High Moderate Low

Conclusion

In summary, crocodilians are well-adapted for an amphibious lifestyle but do not possess gills. Their cardiovascular and respiratory systems allow them to be stealthy underwater hunters while still breathing air.

Key adaptations like valvular nostrils, slow metabolism, and bradycardia enable extended breath-holding when submerged. While sharing some traits with other herptiles, crocodilians have a unique respiratory anatomy fine-tuned for their aquatic ambush hunting behavior.